"During their inquiry, investigators found evidence suggesting the two (suspects) were plotting an attack on the pope," said the source.
An interior ministry statement issued at the time of their deportation said they were being expelled under prevention of terrorism laws.
Six other foreign students, suspected of contacts with militant Islamic groups, are still under investigation.
News magazine Panorama, owned by Prime Minister Silvio Berlusconi's family, reported on Friday that local anti-terrorist police had tapped Hlal's phone and had raised the alarm when he said he wanted to acquire explosives.
The magazine said police discovered a map of Turin at Errahmouni's house annotated with numbers and circles, ahead of a visit to the northern Italian city by Pope Benedict on May 2 to venerate the Shroud of Turin, which many Catholics believe was Jesus Christ's burial cloth.
Panorama described Errahmouni as a computer expert who remained in contact with militant groups over the Internet. It said Perugia had become a center for traveling imams to preach radical Islam.
Turkish citizen Mehmet Ali Agca, who shot and seriously wounded Paul John Paul in 1981, was also enrolled as a language student at Perugia university.
Intelligence reports and arrests show militant Islamic groups linked to Al-Qaeda, especially in North Africa, are active in Italy, mostly recruiting and financing for attacks planned elsewhere in Europe.
However, alarm was raised in October by a failed attack on an army barracks near Milan by a 35-year-old Libyan man. Mohammed Game, whose hand was blown off when he hurled a bomb, was believed to have had accomplices, police said.
